Combined Photoacoustic Ultrasound and Beam Deflection Signal Monitoring of Gold Nanoparticle Agglomerate Concentrations in Tissue Phantoms Using a Pulsed Nd:YAG Laser

Abstract

The purpose of this paper is to show and discuss the effects of the gold nanoparticle (Au-NPs) concentration inside a tissue phantom using a combined system of photoacoustics (PA) and optical beam deflection and their applications particularly to photoacoustic imaging. It was found that the PA signal from aggregated Au nanoparticles is significantly enhanced. The stock concentration of 100 nm Au-NPs was $$3.8\times 10^{9}$$ particles/mL from which three samples with 30 %, 70 %, and 90 % concentration were prepared using polyvinyl chloride-plastisol. Each sample was then irradiated across a line scan using a 10 ns pulsed Q-switched Nd:YAG laser at a 1 Hz repetition rate and $$5~\hbox {W}\cdot \hbox {cm}^{-2}$$ so that no physical ablation was observed. The corresponding photoacoustic pressure was found to approximately cover a range between 10 kPa and 51 kPa. This corresponds to approximately 130 pJ to 315 pJ of acoustic energy radiated by Au-NPs into the tissue. The maximal efficacy of the transformation of optical energy into thermal energy was $${\sim }29~\%$$ . Time-resolved photoacoustic deflection was also used to monitor the laser-interaction process. The results clearly indicated that (i) the photoacoustic signal amplitude varies in a given sample as a result of the non-uniform concentration distribution of embedded Au-NPs; (ii) an increase of the concentration increased the signal amplitude linearly; and (iii) at higher nanoparticle concentrations, the probe deflection was found to increase due to a steeper thermoelastic gradient as a result of a higher absorption by particle agglomerates and particle size-dependent dispersions.